It is known to use a variety of acceleration measuring devices, called accelerometers, particularly for guidance of aircraft, spacecraft, and guided weaponry. A common form of accelerometer is dynamic, employing closed loop feedback to determine accelerations in a sensitive axis. Devices of this type may typically sense accelerations on the order of ten micro g (the acceleration of gravity, 32 ft/sec.sup.2), and are very expensive. In some applications, accelerations on the order of one thousand micro g are involved. This requires a different type of accelerometer. One form of accelerometer known to the prior art is a mass supported on a cantilevered beam, so that acceleration of the mass in the sensitive axis will cause a bending moment in the beam, resulting in a sensible strain. The sensing of strain in the beam has been achieved in a variety of ways. For instance, U.S. Pat. No. 3,411,361 describes the use of bonded resistor bridges disposed on the surfaces of a cavity within the beam to sense the strain therein. However, strain gages of the bonded resistor type typically have sensitivities on the order of two percent of full range, which is inadequate in many applications.
A more sensitive type of cantilevered mass accelerometer employs the variation in propagation time of a surface acoustic wave on a piezoelectric beam, such as quartz. The strain induced by bending alters the acoustic velocity of the wave in the beam, which can be measured in a variety of ways, such as alteration of the frequency of an oscillator in which the frequency determination is principally dependent upon the acoustic velocity of the wave. Such a device is disclosed, inter alia, in U.S. Pat. No. 3,863,497. It is known that devices of this type (when suitably designed) are capable of sensitivities which are at least two orders of magnitude better than the sensitivities of bonded resistor strain sensors. Thus, accelerometers employing SAW devices may have sensitivities on the order of 0.001% of full range.
One characteristic of cantilevered mass accelerometers is the resonant mechanical frequency thereof. It is obvious that the frequency of resonance of the cantilevered beam should be much higher than the equivalent frequency of the accelerations which it is desired to detect thereby. This is essential so that oscillations do not mask the sensing of the desired effect. Additionally, inadvertent inputs to the accelerometer (abrupt accelerations), such as may occur by collision of a space vehicle with a meteorite, may induce oscillations even if the resonant frequency is much higher than normal acceleration inputs. Such oscillations, even if filterable, may cause saturation and otherwise affect the response of the device to the desired effects being sensed.
In closed loop accelerometers, there is no need for a damping medium. In the case of a cantilevered mass, particularly those employing SAW devices to sense the resulting strain, the motion is basically imperceptible in the ranges of accelerations being sensed. Therefore, common forms of damping (such as eddy current and magnetic) are not useful. Furthermore, damping of the beam itself (such as by means of surface wax or organic tape) provides no damping at all to the mass, and such surface damping is of no value at the mass. It is clear that the mass itself has to be damped.
One well known form of damping in inertial devices is fluid damping. Fluid damping not only loads the member being damped, but it dissipates energy by moving fluid from one region to another. However, fluid damping of the mass of a cantilevered mass accelerometer cannot readily be achieved without having fluid adjacent the cantilever itself. When the cantilever comprises a stiff beam in which a surface effect is being measured, such as a SAW device, the fluid will alter or eliminate the surface effect being used to sense the corresponding strain. Thus, fluid damping of cantilevered mass accelerometers would appear to be limited to those which do not employ a surface effect, such as bonded resistor strain sensors. However, as described hereinbefore, the bonded resistor strain sensors which could survive immersion in a damping fluid will not likely provide the desired sensitivity and accuracy for many applications. And, it is impossible to increase that accuracy (such as by increasing the mass) without lowering the resonant frequency to a point where the device would be useless. Additionally, it is known that resistive strain sensors have a high sensitivity to temperature variation, which is very difficult to cause to track identically for cancellation purposes. And, resistive devices, being amplitude analog devices, are not suitable in many applications where digital computation is necessary and real time processing delay constraints preclude conversion of analog amplitude signals to digital form.
One solution to the isolation of a surface device from a damping fluid may appear to be the formulation of the surface acoustic wave devices on surfaces within a chamber formed within the beam itself. However, since the maximal strain as a consequence of acceleration is at the outside surfaces of the beam, and since the strain effects decrease with the distance from the surface, the sensitivity of such a device would be severely diminished. In fact, leaving sufficient wall thickness material for the beam so that it will retain its highly elastic character, with a high mechanical resonant frequency, the SAW devices may in fact end up very near the axis of the beam, and being almost completely insensitive to the desired accelerations.
Therefore, there is a real need in many applications for the features of an accelerometer which are achievable essentially only by the digital, compatible, high sensitive SAW type of strain sensor.